This invention relates to a polypropylene sheet having markedly improved impact strength, in particular high impact resistance at low temperatures, free of anisotropy and having good formability.
Polypropylene sheets are used widely and mainly in the manufacture of a variety of containers, automobile parts, parts of electric devices and appliances, packaging materials, and so forth, either with or without processing such as vacuum forming, pressure forming or press forming. However, polypropylene is unfavorably very poor in impact resistance, in particular impact resistance at low temperatures, and therefore the field of use thereof is limited. Various proposals have already been made to alleviate this difficulty. For instance, there are proposals for improvement in impact resistance according to which polypropylene should be blended with some other resins or plasticizers or polypropylene should be copolymerized with some other monomers. In particular, ethylene-propylene block copolymers are in wide use as propylene-based resins having improved low-temperature impact resistance. However, such chemical modification methods cannot yet produce sufficient improvement in impact strength but rather present difficulties such as decrease in tensile strength, flexural strength and/or elastic modulus. These chemical modification methods further have a drawback in that the modifiers used may easily cause reduction in thermoforming characteristics, processability, electric characteristics, water resistance, solvent resistance, chemical resistance, weatherability and other performance characteristics.
Other methods proposed comprise modification of polypropylene sheet manufacturing conditions or secondary processing of polypropylene sheets obtained. For instance, it is known to uniaxially or biaxially stretch a polypropylene sheet. It is also known to uniaxially or biaxially roll a polypropylene sheet at a temperature lower than the melting point thereof, preferably at a temperature below 140.degree. C. (U.S. Pat. No. 3,714,320). It is further known to extrude a polypropylene sheet in the solid state at a temperature below the crystalline melting point thereof (U.S. Pat. No. 4,282,277). In addition, it is known that solid-phase pressure forming can give a biaxially oriented shaped article or sheet. These methods proposed indeed produce impact resistance-increasing effect to a greater extent as compared with the methods mentioned hereinabove, but the effect is still unsatisfactory. Moreover, the thus-obtained sheets disadvantageously have poor formability and/or shape or dimensional stability, which results from marked orientation of the resin in the crystal region, amorphous region and/or paracrystalline region in the sheet obtained; for instance, when the sheet is preheated for vacuum forming or pressure forming, the tension resulting from said heating may lead to breakage of the sheet or, when stored for a long period, the molded article tends to return to its original shape before molding.